The present invention relates generally to hemodialysis machines. More specifically, the present invention relates to methods and systems for obtaining high whole body dialysis clearance during a dialysis process.
Using dialysis to support a patient whose renal function has decreased to the point where the kidneys no longer sufficiently function is well known. Two principal dialysis methods are utilized: hemodialysis; and peritoneal dialysis. The present application focuses on monitoring the effectiveness of a hemodialysis system.
In hemodialysis, the patient's blood is passed through an artificial kidney or dialyzer. The dialyzer purifies the blood by allowing diffusion of solutes from the blood across the dialyzer membrane into dialysate. Because it is an extracorporeal treatment that requires special machinery, certain factors must be considered when performing hemodialysis.
One of the most basic considerations in treating a patient with hemodialysis revolves around treatment adequacy. Quite simply, how long should a given patient be dialyzed on a given day. A number of medically adverse effects may result from an inadvertent failure to sufficiently dialyze the patient. At the present time, the average dialysis patient has a life expectancy of only about five years.
One reason these patients tend to have a short life expectancy is the deleterious effect of a chronic buildup of various toxins that either are not eliminated at all, i.e. do not pass through the hollow fiber membranes, or are not sufficiently reduced to nontoxic levels. The identity of many of these supposed toxins is not known, although those species known to be eliminated in urine, such as urea, creatinine, phosphate, hydrogen ions, etc. are associated with serious medical consequences when permitted to accumulate in excess of normal levels.
One measure of the adequacy of dialysis for the individual patient during a given treatment is calculated from the following equation: EQU KT/V.gtoreq.1.0
Here K is the urea clearance of the dialysis process in milliliters (ml) of blood cleared of urea each minute. V is an expression of volume of distribution of urea which is approximately equal to the total body water volume. T is the treatment time. K/V is obtained from two blood samples, one taken prior to the start of dialysis and one taken at the end of dialysis. Upon incorporating K/V and T into the above equation, the degree of adequacy of the dialysis process can be determined.
In an effort to increase efficiency of hemodialysis, the industry continually makes further refinements to the hemodialysis process. The performance of a hemodialyzer (BUN clearance) can be affected by several factors related to the patient's vascular access such as: degree of access recirculation, cardiopulmonary recirculation, access flow, and the suction pressure developed by the hemodialysis delivery system blood pump as it pulls blood into the extracorporeal circuit. See Collins, D., et al., Fistula dysfunction: Effect on rapid hemodialysis, Kidney International, 41: 1292-1296 (1992); Daniel, I. D., G. M. Berlyne, and R. H. Barth, Blood flow rate and access recirculation in hemodialysis, The International Journal of Artificial Organs, 15(8): 470-474 (1992); Schneditz, D., et al. Cardiopulmonary Recirculation During Dialysis: Mathematical Analysis, Measurement, and Effects on Computation of Urea Distribution Volume and Access Recirculation in Evolving Dialysis: Advances in Technology, 1992, Beth Israel Medical Center, New York (1992); and Schmidt, D., et al., Inaccurate Blood Flow Rate During Rapid Hemodialysis, American Journal of Kidney Diseases, XVII(1): 34-37 (1991). Parameters that may be varied to achieve adequate dialysis include blood flow rate, dialysis solution flow rate, dialyzer competency and temperature.
Among other parameters, many in the industry have studied the effects of blood flow rate on the performance of a hemodialyzer. Generally, as blood flow increases, clearance increases. Raising the blood flow rate increases dialyzer clearance of small molecular weight solutes, and higher blood flow rates have been more and more widely used to improve dialysis efficiency. However, with conditions like access recirculation, increasing blood flow may actually decrease clearance. See Collins, A., et al., Recirculation and effective clearances, in Am. Soc. of Nephrology; 20th Annual Meeting, Washington, D.C. (1987).
The greater degree of recirculation occurring at higher blood flow rates may neutralize or reverse the increased clearance obtained with the higher blood flow rates. Recirculation of blood during dialysis occurs when insufficient blood flow through the A-V fistula exists in relation to the blood pump flow. This recirculation may be due to either venous stenosis or pump flow rates higher than access flow. Recirculation will reduce the clearance of solutes, resulting in inadequate dialysis in the short-term and potentially long-term detrimental effects on the health of the patient. Hester et al., A New Technique for Determining Recirculation in the ESRD Patient, Nephrology News and Issues, p. 44-45 (June 1993).
Access recirculation of blood flow is normally estimated from three simultaneous measurements of blood urea nitrogen (BUN). Blood samples are taken from the dialyzer inflow line, the dialyzer outflow line, and from a systemic sample. This method has accuracy problems for the true systemic blood sample because of the collection site, and because of the laboratory errors in the BUN measurement. The BUN technique for the determination of recirculation also has several disadvantages: removal of blood from anemic patients; additional exposure of health care workers to blood products; an added cost for the BUN determinations; and the lack of an instantaneous determination of recirculation. See Hester et al., supra.
In light of these problems, a need exists for a system that not only monitors the various factors effecting BUN clearance but also determines the optimum operating conditions to obtain the highest whole body clearance. No simple method currently exists that accounts for the various factors that effect BUN clearance, although repeated blood sampling before and after the dialyzer at different blood flow rates could conceivably yield the desired information. Therefore, providing a non-invasive, on-line monitoring system that determines the optimum parameter settings would be desirable.